Venice. 1866. After a night walking the empty streets of the ancient city together, a countess (Alida Valli) falls in love with an Austrian officer (Farley Granger) and becomes his mistress. War breaks out and separates them until she eventually finds him again in the throes of battle against the Italians. Betraying both her principles and her cause, she tries to reform him with cruel and tragic consequences for them both.